Concentration solar battery protected against heating

ABSTRACT

The invention concerns concentration solar batteries which are protected against heating caused by the fraction of solar radiation which does not enable excitation of the photovoltaic cells ( 101 ) constituting said generator. It consists in covering the concentrator ( 106 ) which reflects the solar flux ( 107 ) towards the photovoltaic cells ( 101 ) with a filter ( 206 ) which enables the useless part of the radiation to be eliminated. It consists in using for that purpose either an absorbent material or an oblique or Fresnel stepped arrangement of the outer surface ( 107 ) of said transparent layer enabling said useless part to be reflected outside the photovoltaic cells ( 101 ).

The present invention relates to photovoltaic generators thatconcentrate incident light and are protected against the effects ofadditional heating caused by this concentration. It applies morespecifically to photovoltaic generators used in artificial satellitesand energized by sunlight.

Using a photovoltaic generator of the kind shown partially in outlinefrom one end in FIG. 1 to supply artificial satellites with electricalenergy is known in the art.

The generator comprises a set of photovoltaic cells 101 covered by atransparent plate 102. The transparent plate protects the surface of thecells and filters received direct solar radiation 103 so as to allowonly wanted radiation 104 to reach the cells and to reflect unwanted(for example infrared and ultraviolet) radiation 105, as the cells areunable to absorb this radiation to produce electricity. In practice thisseparation is imperfect and reflection is not total. A portion of theradiation 105 therefore penetrates the layer 102, where most of it isabsorbed, a small portion reaching the cell, where it is also absorbed,but without producing electricity.

This partial absorption, as much by the layer 102 as by the cell 101,causes additional heating of the assembly, over and above that resultingfrom the normal operation of the cell (Joule effect, various losses).This unwanted heating leads to an increase in the operating temperatureof the cell and therefore to a reduction of the photovoltaic efficiency,because the performance of a cell is degraded as the temperature rises.

Photovoltaic cells are costly and delicate and assembling them intopanels necessitates a structure whose weight is not negligible.Furthermore, the effect of direct cell irradiation fails to achievesaturation in terms of photovoltaic conversion.

Concentrating the sunlight onto the surface of the solar cells coveringit to increase the electrical power supplied by a panel of givendimensions is therefore known in the art. To this end a simple solutionis generally used that consists in surrounding the panel, or morelocally the cells, with inclined plane reflectors like the reflector106. For simplicity, the figure shows only one of these reflectors, butit is standard practice to use several of them, at least two situated oneither side of the panel, or more locally between the rows of cells onthe panel.

The solar flux 107 impinging on the concentrator is reflected toward thesurface of the layer 102 in the form of a reflected flux 108. As in thecase of the direct flux 103, the wanted portion of the reflectedradiation penetrates into the layer 102 in the form of a flux 109 toexcite the cell 101. The other portion is reflected in the form of aflux 110. The effects of the flux coming from the concentrator are thesame as those of the direct flux and therefore lead to additionalheating of the solar panel, in direct proportion to the concentrationobtained.

This additional heating leads to a reduction in photovoltaic conversionefficiency, because the performance of solar cells is degraded if theiroperating temperature rises. This phenomenon therefore counteracts tosome extent the benefit of using a concentrator.

Furthermore, the concentrators 106 consist of simple, generallymetallic, reflective surfaces, in order to be as light as possible.These surfaces absorb virtually none of the incident flux 107 andreflect all of it in the form of the reflected flux 108.

This being so, the operating temperature of the concentrators 106 is acold temperature.

The concentrators therefore become large cold traps for all circulatingmolecules, and their surface is rapidly polluted, which leads to asignificant reduction in their reflective efficiency, and in due courseto a considerable reduction in the efficiency of the photovoltaicgenerator as a whole.

To overcome these drawbacks, the invention proposes a concentratorphotovoltaic generator, comprising at least one photovoltaic cellcovered by a transparent protection layer and a reflecting concentrator,characterized in that the concentrator is covered by a filter toeliminate in the luminous flux reflected by the concentrator toward thephotoelectric cell most of the “unwanted” radiation that is not able toexcite the photovoltaic cell.

According to another feature of the invention, the filter is formed of alayer made from materials absorbing the “unwanted” portion of theradiation.

According to another feature of the invention, the layer forming thefilter is of constant thickness.

According to another feature of the invention, the filter is formed of alayer whose exterior face is oriented to divert this “unwanted”radiation out of the voltaic cell.

According to another feature of the invention, the transparent layer isof decreasing thickness so that its exterior face is not parallel to thereflecting surface of the concentrator.

According to another feature of the invention, the exterior face of thetransparent layer forming the filter is etched to form Fresnel steps.

Other features and advantages of the invention will become clearlyapparent in the course of the following description, which is given byway of nonlimiting example and with reference to the appended drawings,in which:

FIG. 1 is an end view of a prior art generator;

FIG. 2 is a similar view of a first embodiment of a generator accordingto the invention; and

FIG. 3 is a similar view of a second embodiment of a generator accordingto the invention.

The invention therefore consists in disposing on the concentrator panel106 a filter that limits the radiation reflected toward thephotoelectric cells essentially to the wavelengths that they are able touse. In one variant, the radiation at the wavelengths eliminated in thisway is absorbed in the concentrator to heat it to prevent it becoming acold trap.

In a first embodiment, shown in FIG. 2, the concentrator 106 is coveredwith a transparent layer 206 whose exterior face 116 is inclined to theplane of the reflecting face of the concentrator 106 so that theluminous flux 107 is divided into two portions. A first portion 207,corresponding to the wavelengths used for photoelectric conversion,penetrates the filter, is reflected by the concentrator 106 in the formof a flux 217, and then emerges from the inclined face of the filter206, where it is refracted to form a beam 208 directed toward the upperface of the transparent layer 102 that protects the cells 101.

A second portion 218 of the flux 107, corresponding to the wavelengthsthat are not able to be used for photoelectric conversion, is subject tototal reflection at the upper face of the filter 206 to form a flux 218that is directed out of the photoelectric device.

The wanted wavelengths of the flux 208 are refracted as flux 209 toexcite the cells 101. Given inevitable imperfections and transitioneffects, the flux 208 nevertheless contains a certain percentage ofunwanted wavelengths, a portion of which is reflected in the form of aflux 210 and a residual portion of which contributes to unwanted heatingof the cells 101. However, this effect is reduced compared to theabsence of any filter.

The necessary inclinations of the concentrator 106 relative to the cells101 and of the external face 116 of the filter 206 relative to theconcentrator are chosen so that there is total reflection of theunwanted wavelengths at the inclined face 116 (remember that refractionon passing from one medium to another, and thus total reflection whereapplicable, depends on the wavelength of the light rays, which enablesthis separation), and so that the combination of reflection at theconcentrator 106 and refraction on passing through the face 116 directsthe wanted wavelengths toward the external surface of the transparentlayer 102.

In the example shown in this figure, the filter 206 takes the form of arelatively thick plate whose thickness reduces from one end to the otherof the surface of the concentrator 106 to obtain the requiredinclination. This causes a relatively large increase in the weight ofthe assembly, which is not necessarily desirable.

In a variant, the filter 206 is formed of a refractive transparent layerwhose average thickness is substantially constant and as small aspossible. To obtain the desired effect in this case, the exterior face116 of this layer is machined to form Fresnel steps, to obtain therequired effect locally whilst limiting the overall thickness of thefilter.

In a second embodiment, shown in FIG. 3, a filter 306 is placed on thereflecting surface of the concentrator 106 and takes the form of a planeplate of uniform thickness. This plate is made from a material thatabsorbs “unwanted” components of the incident solar flux 107 but istransparent to “wanted” components of the same flux (those enablingphotoelectric conversion to be obtained in the cells 101).

A solid absorbent material may be used for this purpose, as is known inthe art, and/or a combination of thin reflecting layers with differentindices, as is also known in the art.

In the embodiment shown, the angles and the refractive index of thematerial constituting the layer 306 are chosen so that the path 307 ofthe solar flux in the layer 306 is of the same length in bothdirections, reflection at the concentrator 106 occurring in a directionnormal to the surface thereof. This is merely one particular example andthese paths could be different, as in FIG. 2.

On leaving the filter 306, the solar flux 308 is therefore for the mostpart freed of “unwanted” components and travels along a path 309 causedby refraction until it impinges on the exterior surface of thetransparent layer 102 to excite the cells 101.

As in the first embodiment, a portion of this “unwanted” flux isreflected at the surface of the transparent layer 102 to form a flux 310that is lost in space, and only a vanishingly small portion of this“unwanted” radiation is contained in the flux 309 and makes a very smallcontribution to unwanted heating of the cells 101.

The separation of the “wanted” flux and the “unwanted” flux may be lessin this second embodiment, which is nevertheless preferred since anadditional advantage is obtained, namely heating of the concentrator 106and its filter 306. This increased operating temperature greatly limitsthe trapping of molecules and unwanted particles encountered in space,and the reflective power of the concentrator therefore remainspractically constant.

1. A concentrator photovoltaic generator, comprising at least onephotoelectric cell covered by a transparent protection layer and furthercomprising a reflecting concentrator for directing luminous flux towardsaid at least one photoelectric cell, said concentrator having areflecting surface for reflecting incident radiation, characterized inthat the reflecting surface of said concentrator is covered by a filtersuch that the incident radiation must pass through said filter to reachsaid reflecting surface in order to be reflected, and after reflectionby said reflecting surface must pass again through said filter in orderto be directed toward said at least one photoelectric cell, said filtereliminating in the luminous flux directed by the concentrator towardsaid at least one photoelectric cell most of “unwanted” radiation thatis not able to excite said at least one photoelectric cell.
 2. Theconcentrator photovoltaic generator according to claim 1, characterizedin that the filter is formed of a filter layer made from materialsabsorbing the “unwanted” radiation.
 3. The concentrator photovoltaicgenerator according to claim 2, characterized in that the filter layeris of constant thickness.
 4. The concentrator photovoltaic generatoraccording to claim 1, characterized in that the filter is formed of afilter layer whose exterior face is oriented to divert the “unwanted”radiation away from said at least one photoelectric cell.
 5. Theconcentrator photovoltaic generator according to claim 4, characterizedin that the filter layer is of decreasing thickness so that the exteriorface is not parallel to the reflecting surface of the concentrator. 6.The concentrator photovoltaic generator according to either claim 4,characterized in that the exterior face of the filter layer is etched toform Fresnel steps.
 7. The concentrator photovoltaic generator accordingto claim 1, wherein said filter is formed of a material reflecting the“unwanted” radiation.